1. Field of the Invention
The invention relates to a device for drying a continuous web. More particularly, the present invention relates to a device for drying paper, in particular hygienic papers, in which the web dries under heat and pressure applied by a drying cylinder.
2. Background and Material Information
Devices which dry a continuous web are known. In these prior art devices, at least one drying cylinder guides the web. A steady supply of steam to the core of the drying cylinder heats the periphery (sleeve) of the cylinder. When the web is guided by a press roll/section over the rotating drying cylinder, the heat from the cylinder dries the web.
For cost-effectiveness, it is crucial that as much heat as possible transmits from the cylinder core through the sleeve to optimize the use of heat from the hot steam. To improve heat transmission, the inner surface of the drying cylinder has a series of circumferential grooves which are separated from one another by ribs. The decrease in wall thickness of the sleeve at the grooves improves the overall transmission of heat from the interior of the cylinder to its outer surface, and thus to the moving web.
As the steam cools inside the drying cylinder, condensation collects on the inner surface of the grooves, and is removed in a suitable manner. However, a layer of condensation usually remains on the inner surface from which the ribs project to further conduct heat from the cylinder to the outer surface.
In these prior art devices, at least one contact-press roll contacts the drying cylinder under a predetermined pressure, the web is fed between the drying cylinder and such a contact-press roll. The applied pressure is preferably high to improve the drying process and to optimize the cost-effectiveness of the web manufacturing machine. However, the presence of the grooves weakens the flextional strength of the drying cylinder such that it cannot withstand the application of such high pressure from the contact-press roll, particularly when the contact pressure has a line force of greater than 90 kN/m.
Although an increase in sleeve thickness of the drying cylinder provides greater strength, the corresponding loss in heat transfer through the thicker sleeve offsets any such beneficial results. To date, to meet production levels of 2000 m/min, the only acceptable solution is to use extremely large drying cylinders, on the order of 5.5 m in diameter. Drying cylinders of this size are expensive and difficult to transport.